The number of law enforcement officers killed by firearms jumped by 56 percent this year and included 15 ambush deaths. But gun-related police deaths still remain far below historic highs and lower than the average annual figures in the past decade, according to a report released Tuesday.

The parents of Michael Brown and Eric Garner, two black men who died in encounters with white police officers, joined with civil rights leaders on Thursday to call for a full federal investigation and charges against those involved in their deaths.